Not sure how much of the evidence of climate change you can trust? Several independent records agree: Earth is warming, and human activity is the main cause.
This guide is one part of our wider coverage of climate change, and it focuses on how scientists know what they know.
| Item | Value |
|---|---|
| Rise in average surface temperature since 1880 | About 1.8°F (1.0°C) |
| Share of Earth’s fresh water held by the Greenland ice sheet | About 8 percent |
| Atmospheric CO2 over the past 150 years | From 280 ppm to 414 ppm |
| Current warming versus ice-age-recovery warming | Roughly 10 times faster |
| Human CO2 increase versus natural increase after the last ice age | More than 250 times faster |
| Climate scientists who agree on human-caused warming | 97 percent or more of actively publishing scientists |
The figures come from NOAA Climate.gov’s Q&A, published October 12, 2022 and archived as of June 25, 2025, and from the Caltech Science Exchange. Figures can change as records are updated, so check the source before you rely on them.
What the temperature record shows
The most direct temperature record comes from thousands of weather stations over land and ocean, some of which have run for more than a century. When separate teams at agencies such as NOAA and NASA, and at the U.K.’s Hadley Centre, average those readings, they all find essentially the same result: Earth’s average surface temperature has risen by about 1.8°F (1.0°C) since 1880.
That agreement matters because the teams sit in different agencies and countries. The finding does not rest on one group’s choices.
NASA’s evidence page states that there is unequivocal evidence that Earth is warming at an unprecedented rate, and that human activity is the principal cause. It adds that the current warming is happening at a rate not seen in the past 10,000 years.
Signs in ice, oceans and living things
A thermometer is only one line of evidence. NOAA’s Climate.gov Q&A points to a wider set of independent signs that all move in the same direction.
According to NOAA, researchers see the following changes:
- Birds are migrating earlier, and their migration patterns are changing.
- Lobsters and other marine species are moving north.
- Plants are blooming earlier in the spring.
- Mountain glaciers are melting worldwide, and snow cover is declining in the Northern Hemisphere.
- Arctic sea ice is declining rapidly in both thickness and extent.
NOAA also reports that Greenland’s ice sheet, which holds about 8 percent of Earth’s fresh water, is melting at an accelerating rate, and that mean global sea level is rising. Our guide to sea level rise goes deeper into how that rise is measured.
The Caltech Science Exchange adds a warming ocean, ocean acidification and extreme weather events to the list of observable evidence. Taken together, the signs on land, at sea and in wildlife make a case that no single dataset has to carry alone.
How do we know carbon dioxide is behind it?
The greenhouse effect is the starting point. The National Academies explain that greenhouse gases absorb and emit heat energy in all directions, including downwards, which keeps the surface and lower atmosphere warm. Without this effect, life as we know it could not have evolved, but adding more of these gases makes the atmosphere even more effective at preventing heat from escaping.
NOAA notes that the key role carbon dioxide plays in the greenhouse effect has been understood since the mid-1800s. Unless an equally large cooling influence offsets it, more atmospheric CO2 leads to warmer surface temperatures.
The measured rise is large. Caltech reports that CO2 levels have increased from 280 parts per million (ppm) to 414 ppm in the past 150 years. The National Academies add that, since preindustrial times, three gases have gone up by the following amounts:
- Carbon dioxide: over 40%.
- Methane: more than 150%.
- Nitrous oxide: roughly 20%.
The same source says more than half of the increase in CO2 has occurred since 1970, and that the CO2 increase plays the largest role in warming.
Scientists can also tell where the extra CO2 came from. NOAA says the rapid increase and the carbon’s isotopic fingerprint show the source is fossil fuels, and not natural sources like forest fires, volcanoes, or outgassing from the ocean. To see how everyday activity adds to those emissions, read our explainer on your carbon footprint.
Why natural cycles do not explain the warming
Earth’s climate has changed before, and scientists study those changes closely. NASA notes that in the last 800,000 years there have been eight cycles of ice ages and warmer periods, and that the last ice age ended about 11,700 years ago. Most of those swings are attributed to very small variations in Earth’s orbit that change the amount of solar energy the planet receives.
Ice cores give a long baseline for carbon dioxide. The National Academies report that ice core records extending back 800,000 years show CO2 staying within 170 to 300 ppm through many ice-age cycles. No concentration above 300 ppm appears in those records until the past 200 years.
Speed is the other clue. According to Caltech, current warming is occurring roughly 10 times faster than the average rate of ice-age-recovery warming, and CO2 from human activity is increasing more than 250 times faster than it did from natural sources after the last ice age.
Researchers have checked the other suspects, and none fits:
- Earth’s orbit: the warming of recent decades is too rapid to be linked to orbital changes.
- Solar activity: the warming is too large to be caused by changes in the Sun.
- Other known influences: NOAA says no other known climate influences have changed enough to account for the observed warming trend.
Where scientists agree and how to read the claims
Caltech’s summary says multiple peer-reviewed studies show that 97 percent or more of actively publishing climate scientists agree that warming over the past century is extremely likely due to human activities. As of the IPCC’s Synthesis Report of March 2023, human activities, principally through greenhouse gas emissions, have unequivocally caused global warming.
NASA quotes the IPCC on how the field has moved: since systematic scientific assessments began in the 1970s, the influence of human activity on warming has evolved from theory to established fact. In other words, the consensus was built over decades of evidence, not announced overnight.
When you read a claim about the climate, a few habits help:
- Look for independent records, such as thermometers, ice cores and satellites, that point the same way.
- Separate evidence that the planet is warming from evidence about what is causing it.
- Check the date of the source, because agencies update their datasets and summaries.
NASA also notes that information from natural sources such as ice cores, rocks and tree rings, and from modern equipment such as satellites and instruments, all show the signs of a changing climate.
Frequently asked questions
Is climate change just a natural cycle?
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Natural cycles, mostly tied to small variations in Earth’s orbit, explain earlier ice ages. Caltech says the warming of the past few decades is too rapid to be linked to orbital changes and too large to be caused by solar activity.
How much has Earth warmed?
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NOAA reports that Earth’s average surface temperature has risen by about 1.8°F (1.0°C) since 1880, based on records that different teams have averaged and compared. Check the NOAA page for its wording and date.
What is the most direct evidence that humans are responsible?
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The rise in CO2 is central. Caltech reports a climb from 280 ppm to 414 ppm in the past 150 years, and NOAA says the isotopic fingerprint points to fossil fuels rather than natural sources.
How do scientists know about climate before thermometers?
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They use ancient, or paleoclimate, evidence. According to Caltech, ice cores from Greenland, Antarctica and tropical mountain glaciers show that Earth’s climate responds to greenhouse gas levels, and tree rings, ocean sediments, coral reefs and layers of sedimentary rocks add more clues.
